Effect of dietary β-glucan levels on the growth response and efficacy of Streptococcus iniae vaccine in Nile tilapia, Oreochromis niloticus

A feeding study was conducted to determine the effect of dietary β-glucan levels on growth performance, immune responses and resistance of immunized and non-immunized Nile tilapia to Streptococcus iniae challenge. Fish were fed a 32% protein commercial diet supplemented with 0, 50, 100 and 200 mg β-glucan for 14 weeks. After 10 weeks of feeding, neither immunization, β-glucan nor their interaction affected weight gain and survival. Feed efficiency ratio (FER) of fish fed the 100 and 200-mg β-glucan diets was slightly but significantly lower than those of the group fed the 50-mg β-glucan diet, but these values did not differ from that of the control (0 mg β-glucan diet). Average percent mortality (PM) decreased and relative percent survival (RPS) increased in fish immunized with the S. iniae ARS-98-60 vaccine. Dietary levels of β-glucan and the interaction between dietary levels of β-glucan and immunization had no effect on PM and RPS. Serum lysozyme activity at 14 days post-challenge (PC) significantly decreased in fish fed the highest dietary β-glucan level but was not affected by immunization and the interaction between β-glucan and immunization. Specific antibody levels 21 days post-booster immunization (PBI) and 14 days PC were not affected by dietary β-glucan and the interaction between β-glucan and immunization These parameters, however, were significantly enhanced by immunization with S. iniae vaccine, leading to increase protection against S. iniae infection. Dietary levels of β-glucan had no effect on stimulating the S. iniae-specific antibody response and resistance of Nile tilapia against S. iniae infection.